These two bright debris disks of ice and dust appear to be theequivalent of our own solar system's Kuiper Belt, a ring of icy rocksoutside the orbit of Neptune and the source of short-period comets. Thedisks encircle the types of stars around which there could be habitablezones and planets for life to develop. The disks seem to have a centralarea cleared of debris, perhaps by planets.

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This paper considers the distribution of dust which originates in the break-up of planetesimals that are trapped in resonance with a planet. There are three grain populations with different spatial distributions: (I) large grains have the clumpy resonant distribution of the planetesimals; (II) moderate sized grains are no longer in resonance and have an axisymmetric distribution; (III) small grains are blown out of the system by radiation pressure and have a distribution which falls off oc 1/r, however these grains can be further divided into subclasses: (IIIa) grains produced from pop I that exhibit trailing spiral structure emanating from the clumps; and (IIIb) grains produced from pop II that have an axisymmetric distribution. Since observations in different wavebands are sensitive to different sized dust grains, multi-wavelength imaging can be used to test models for the origin of debris disk structure. For example, a disk with no blow-out grains would appear clumpy in the sub-mm, but smooth at mid- to far-IR wavelengths. The wavelength of the transition is indicative of the mass of the perturbing planet. The size distribution of Vega's disk is modeled in the light of the recent Spitzer observations. The origin of the large quantities of pop III grains seen by Spitzer must be in the destruction of the grains seen in the sub-mm, and so at high resolution and sensitivity the mid- to far-IR structure of Vega's disk is predicted to include spiral structure emanating from the sub-mm clumps.

Authors: C. A. Beichman, A. Tanner, G. Bryden, K. R. Stapelfeldt, M. W. Werner, G. H. Rieke, D. E. Trilling, S. Lawler, T. N. Gautier\\ We report the results of a spectroscopic search for debris disks surrounding41 nearby solar type stars, including 8 planet-bearing stars, using the {\itSpitzer Space Telescope}. With accurate relative photometry using the InfraredSpectrometer (IRS) between 7-34 $\micron$ we are able to look for excesses assmall as $\sim$2% of photospheric levels with particular sensitivity to weakspectral features. For stars with no excess, the $3\sigma$ upper limit in aband at 30-34 $\mu$m corresponds to $\sim$ 75 times the brightness of ourzodiacal dust cloud. Comparable limits at 8.5-13 $\mu$m correspond to $\sim$1,400 times the brightness of our zodiacal dust cloud. These limits correspondto material located within the $<$1 to $\sim$5 AU region that, in our solarsystem, originates from debris associated with the asteroid belt. We findexcess emission longward of $\sim$25 $\mu$m from five stars of which four alsoshow excess emission at 70 $\mu$m. This emitting dust must be located around5-10 AU. One star has 70 micron emission but no IRS excess. In this case, theemitting region must begin outside 10 AU; this star has a known radial velocityplanet. Only two stars of the five show emission shortward of 25 $\micron$where spectral features reveal the presence of a population of small, hot dustgrains emitting in the 7-20 $\mu$m band. The data presented here strengthen theresults of previous studies to show that excesses at 25 $\micron$ and shorterare rare: only 1 star out of 40 stars older than 1 Gyr or $\sim 2.5$% shows anexcess. Asteroid belts 10-30 times more massive than our own appear are rareamong mature, solar-type stars.

We present the first scattered light images of debris disks around a K star (HD 53143) and an F star (HD 139664) using the coronagraphic mode of the Advanced Camera for Surveys (ACS) aboard the Hubble Space Telescope (HST). With ages 0.3 - 1 Gyr, these are among the oldest optically detected debris disks. HD 53143, viewed ~45 degrees from edge-on, does not show radial variation in disk structure and has width >55 AU. HD 139664 is seen close to edge-on and has belt-like morphology with a dust peak 83 AU from the star and a distinct outer boundary at 109 AU. We discuss evidence for significant diversity in the radial architecture of debris disks that appears unconnected to stellar spectral type or age. HD 139664 and possibly the solar system belong in a category of narrow belts 20-30 AU wide. HD 53143 represents a class of wide disk architecture with characteristic width >50 AU.

Comments: 10 pages, 14 figures, MNRAS, submitted\\ The binary star system gamma Cephei is unusual in that it harbours a stablegiant planet around the larger star at a distance only about a tenth of that ofthe stellar separation. Numerical simulations are carried out into thestability of test particles in the system. This provides possible locations foradditional planets and asteroids. To this end, the region interior to theplanet is investigated in detail and found to permit structured belts ofparticles. The region between the planet and the secondary star however showsalmost no stability. The existence of an Edgeworth-Kuiper belt analogue isfound to be a possibility beyond 65 au from the barycentre of the system,although it shows almost no structural features. Finally, the region around thesecondary star is studied for the first time. Here, a zone of stability is seenout to 1.5 au for a range of inclinations. In addition, a ten Jupiter-massplanet is shown to remain stable about this smaller star, with the habitabilityand observational properties of such an object being discussed.

Astronomers estimate that the stars' disks are also bloated, spreading all the way out to an orbit about 60 times more distant than Pluto's around the sun. The disks are probably loaded with about ten times as much mass as is contained in the Kuiper Belt. Kastner and his colleagues say these dusty structures might represent the first or last steps of the planet-forming process. If the latter, then the disks can be thought of as enlarged versions of our Kuiper Belt.

"These disks may be well-populated with comets and other larger bodies called planetesimals," said Kastner. "They might be thought of as Kuiper Belts on steroids."

Spitzer detected the disks during a survey of 60 bright stars thought to be wrapped in spherical cocoons of dust. According to Kastner, R 66 and R 126 "stuck out like sore thumbs" because their light signatures, or spectra, indicated the presence of flattened disks. He and his team believe these disks whirl around the hypergiant stars, but they say it is possible the giant disks orbit unseen, slightly smaller companion stars.

SigurRosFan

Feb 8 2006, 10:42 PM

Whoa! These are the first extragalactic disks??

SigurRosFan

Feb 9 2006, 01:34 PM

These two stars are not hypergiants (Ia-0)!

R126 = HD 37974 = B0.5 Ia = (bright supergiant)

R66 = HD 268835 = B8 Ia (bright supergiant) = exactly like Rigel

LMC overlook:

ljk4-1

Feb 9 2006, 02:09 PM

QUOTE (SigurRosFan @ Feb 9 2006, 08:34 AM)

These two stars are not hypergiants (Ia-0)!

R126 = HD 37974 = B0.5 Ia = (bright supergiant)

R66 = HD 268835 = B8 Ia (bright supergiant) = exactly like Rigel

Plain ol' supergiant stars just don't cut it any more. Just like some supernovaeare now hypernovae!

Astronomers estimate that the stars' disks are also bloated, spreading all the way out to an orbit about 60 times more distant than Pluto's around the sun. The disks are probably loaded with about ten times as much mass as is contained in the Kuiper Belt. Kastner and his colleagues say these dusty structures might represent the first or last steps of the planet-forming process. If the latter, then the disks can be thought of as enlarged versions of our Kuiper Belt.

"These disks may be well-populated with comets and other larger bodies called planetesimals," said Kastner. "They might be thought of as Kuiper Belts on steroids."

Spitzer detected the disks during a survey of 60 bright stars thought to be wrapped in spherical cocoons of dust. According to Kastner, R 66 and R 126 "stuck out like sore thumbs" because their light signatures, or spectra, indicated the presence of flattened disks. He and his team believe these disks whirl around the hypergiant stars, but they say it is possible the giant disks orbit unseen, slightly smaller companion stars.

That illustration on the Spitzer site is nice...but couldn't they provide an actual image of the giants? Don't tease us like that

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